Pipe-laying vessel and method of laying a pipeline

ABSTRACT

A pipe-laying vessel ( 1 ) is provided, including a tower ( 2 ) extending upwardly from the vessel, a pipe loading arm ( 9 ) for raising a length of pipe from the deck to a position aligned with the tower. The pipe-laying vessel may also include a support assembly ( 110 ) for supporting a bulky item to be accommodated along the pipeline, the support assembly including a frame ( 111 ) pivotable between a first load receiving position and a second tilted position in which it extends approximately parallel to the tower, and drive means for pivoting the tiltable frame between its first and second positions. The pipe-laying vessel may alternatively or additionally include a welding assembly station cabin ( 100 ) in the region of a lower portion of the tower, a movable support assembly ( 110 ) for supporting a bulky item to be accommodated along the pipeline, the movable support assembly being disposed on top of the cabin.

TECHNICAL FIELD

This invention relates to a pipe-laying vessel and to a method of layinga pipeline. Such a vessel and method is of particular use in theoffshore industry relating to oil and gas production.

BACKGROUND OF THE INVENTION

One of the tasks that is carried out by certain vessels in the offshoreindustry is that of laying pipelines, either by S-laying or by J-laying.J-laying, in which the pipeline is lowered down an inclined or verticaltower and curves round in a single direction to become approximatelyhorizontal along the seabed, is used primarily for laying pipeline indeep water (that is typically deeper than 1,000 m). Examples of J-layingarrangements are shown in U.S. Pat. No. 6,524,030, U.S. Pat. No.6,592,297, WO 2006/054891, WO 2005/085692 and U.S. Pat. No. 6,796,742.

SUMMARY OF INVENTION

It is an object of the invention to provide an improved pipe-layingvessel and an improved method of laying a pipeline.

According to a first aspect of the invention there is provided apipe-laying vessel including a tower extending upwardly from the vessel,a pipe loading arm for raising a length of pipe from the deck to aposition aligned with the tower, a support assembly for supporting abulky item to be accommodated along the pipeline, the support assemblyincluding a frame pivotable between a first load receiving position anda second tilted position in which it extends approximately parallel tothe tower, and drive means for pivoting the tiltable frame between itsfirst and second positions.

By providing a tiltable frame to receive the bulky item, the streamlinedintroduction of the bulky item into the pipeline is facilitated withonly modest amounts of further equipment required.

It should be understood that the pipe loading arm and the supportassembly for supporting the bulky item are separate items of equipment.

Preferably the tiltable frame extends approximately horizontally when itis in the first load receiving position. In that case loading of thebulky item onto the tiltable frame, either by lowering it from above orby skidding it sideways onto the frame, is facilitated.

The support assembly is preferably a movable support assembly and mayinclude a movable support on which the tiltable frame is pivotallymounted. The movable support assembly is preferably movable towards andaway from the tower. That allows the bulky item to be loaded onto thetiltable frame and, after loading, moved towards the tower.

Preferably the vessel further includes a welding assembly station cabinin the region of a lower portion of the tower. The support assembly ispreferably disposed on top of the cabin. The cabin is preferably anenclosed space which may provide weather protection but it may also bean open frame construction. In the case where it defines an enclosedspace it preferably defines a single compartment, but it mayalternatively define two or more smaller compartments.

Preferably at least part of the support assembly rests on and issupported by the cabin. That enables a more compact and lightweightarrangement to be provided.

Preferably the cabin is movable towards and away from the tower. In thatcase, the cabin may be interposed along the pipe-laying path (firingline) during welding of a new pipe length (also referred to herein as apipe string) to the pipeline but may be retracted to a position clear ofthe pipe-laying path at other times.

The movable support assembly may be movable in unison with the cabinand/or it may be movable independently of the cabin. In an embodiment ofthe invention described below the assembly is movable both in unisonwith the cabin and independently of the cabin.

Preferably the cabin and/or the movable support assembly are mounted onrails. More preferably, the cabin and the movable support assembly aremounted at least partly on a common set of rails. In an embodiment ofthe invention described below, the cabin and one end of the movablesupport assembly are mounted on a common set of rails and the other endof the movable support assembly is mounted on rails on the roof of thecabin.

Preferably the movable support comprises a first frame, the tiltableframe defining a second frame pivotally mounted on the first frame andpivotable between the first load receiving position and the secondtilted position.

The drive means for pivoting the tiltable frame may take any of a widevariety of forms including a rack and pinion drive and a winch and ropedrive, but in an embodiment described below the drive means comprisesone or more hydraulic cylinders (jacks).

The invention may be applied to a wide range of pipe-laying systemsincluding ones with a fixed tower and ones where the inclination of thetower can be varied, for example systems where the inclination of thetower can be varied by an angle of more than 30 degrees. Also theinvention may be applied to a tower in which there is a travelling clampassembly which may for example be lowered down the tower to lower thepipeline into the sea, or to a tower with one or more tensioners fixedon the tower for lowering the pipeline at a controlled rate. A fixedclamp assembly, also known as a hang off clamp assembly, is preferablyprovided in the region of a lower portion of the tower fixed to thevessel or to the tower. In an embodiment of the invention describedbelow, the tower is pivotally mounted and is able to operate over arange of angles to the horizontal; in the particular example describedthe tower can operate at angles in the range of 45 degrees to 96 degreesto the horizontal. In the embodiment described a travelling clampassembly is mounted for movement along the tower and a fixed clampassembly is located in the region of a lower portion of the tower and inthe particular example described is mounted on the tower.

In a preferred arrangement, the pipe loading arm is raised to a positionalong the tower spaced away from the centre of the tower. At that stagethe pipe length carried by the pipe loading arm is preferably well clearof the pipe-laying path (firing line). The pipe string carried by thepipe loading arm can then be swung around the arm to a position alongthe centre of the tower, that is equidistant from the opposite sides ofthe tower.

A stinger or similar structure may be provided below the tower. Thestinger may control the curvature of the pipeline as it leaves thetower. The stinger may be retractable.

The axis of pivoting of the tiltable frame is preferably in the regionof the tower. In an embodiment of the invention described below, theaxis of pivoting is in the region of the end of the first frame nearerto the tower. Preferably the location of the axis of pivoting isadjustable, for example to suit different inclinations of the towerand/or to accommodate different shapes of bulky item.

The provision of a movable support assembly for a bulky item on top ofthe cabin is itself a special and inventive feature. Accordingly, in asecond aspect, the invention provides a pipe-laying vessel including atower extending upwardly from the vessel, a pipe loading arm for raisinga length of pipe from the deck to a position aligned with the tower, awelding assembly station cabin in the region of a lower portion of thetower, a movable support assembly for supporting a bulky item to beaccommodated along the pipeline, the movable support assembly beingdisposed on top of the cabin.

By providing the movable support assembly on top of the cabin, thestreamlined introduction of the bulky item into the pipeline is againfacilitated with only modest amounts of further equipment required.

According to a third aspect of the invention, there is provided apipe-laying vessel including a tower extending upwardly from the vessel,a pipe loading arm for raising a length of pipe from the deck to aposition aligned with the tower, wherein a rope and winch system isprovided for moving the pipe loading arm to raise the length of pipe,the rope and winch system including two ropes operating in parallelwhereby movement of the loading arm can still be effected by the systemin the event that one of the two ropes is inoperative.

The operation of the loading arm is a key part of the pipe-layingprocedure and if interrupted, can prevent any pipe-laying. By providinga dual rope system, where even if one rope is severed, the loading armcan still be operated, although possibly at reduced speed, thereliability of pipe-laying is significantly improved.

The pipe loading arm is preferably pivotally mounted about a pivot inthe region of the bottom of the tower for allowing the loading arm topivot between a lower, substantially horizontal position and a raisedposition in which the loading arm is substantially parallel to thelongitudinal axis of the tower.

The two ropes preferably follow adjacent paths. They may be wound on thesame winch drum or adjacent winch drums driven by the same drive.

Each rope may include an extensible element along its length. Such anelement facilitates an equal sharing of loads between the two ropesduring normal operation when both ropes are operative.

A vessel of any of the kinds defined above can be designed to operate atconsiderable depths with correspondingly high tension loads. Forexample, the equipment on the vessel is preferably able to handle pipetension loads over 1,000 tonnes and more preferably over 1,500 tonnes,and preferably loads of 2,000 tonnes.

According to the first aspect of the invention, there is provided amethod of laying a pipeline from a pipe-laying vessel, the methodcomprising the following steps:

providing a vessel with an upwardly extending tower,

raising a length of pipe on a pipe loading arm from a deck of the vesselto a position aligned with the tower,

welding the length of pipe to the end of a pipeline suspended from thevessel,

lowering the pipeline with the length of pipe welded thereto,

resting a bulky item to be added to the pipeline on a tiltable frame,

tilting the frame to a position extending approximately parallel to thetower,

transferring the support for the bulky item from the tiltable frame toone or more devices in the tower, and

connecting the bulky item to the pipeline suspended from the vessel.

The tiltable frame preferably extends approximately horizontally when itreceives the bulky item.

The welding of the length of pipe to the end of the pipeline ispreferably carried out in a welding assembly station cabin disposed inthe region of a lower portion of the tower. In that case, the tiltableframe is preferably tilted from a position overlying the top of thecabin to the position extending approximately parallel to the tower.

The method preferably further includes the step of moving the cabintowards the tower. The cabin and the support assembly are preferablymoved towards the tower in unison. More preferably the support assemblyis moved towards the tower and the cabin in a first step and is moved inunison with the cabin towards the tower in a subsequent step.

According to the second aspect of the invention, there is provided amethod of laying a pipeline from a pipe-laying vessel, the methodcomprising the following steps:

providing a vessel with an upwardly extending tower,

raising a length of pipe on a pipe loading arm from a deck of the vesselto a position aligned with the tower,

welding the length of pipe to the end of a pipeline suspended from thevessel, the welding being carried out in a welding assembly stationcabin disposed in the region of a lower portion of the tower,

lowering the pipeline with the length of pipe welded thereto,

resting a bulky item to be added to the pipeline on a movable supportassembly disposed on top of the cabin,

moving the movable support assembly towards the tower,

transferring the bulky item from the movable support assembly into thetower, and

connecting the bulky item to the pipeline suspended from the vessel.

According to a fourth aspect of the invention, there is provided apipe-laying vessel including a tower extending upwardly from the vessel,a pipe loading arm for raising a length of pipe from the deck to aposition aligned with the tower, a travelling clamp assembly mounted formovement along the tower, a fixed clamp assembly located in the regionof a lower portion of the tower, wherein the travelling clamp assemblyand/or the fixed clamp assembly includes both a friction clamp and acollar clamp, each of the friction clamp and the collar clamp beingmovable between an operative position, in which it is adjustable betweena clamping and a released position, and an inoperative position.

By providing both a friction clamp and a collar clamp, each movablebetween operative and inoperative positions, an especially versatileclamping arrangement can be provided.

Preferably each of the friction clamp and the collar clamp isindependently movable between the operative and inoperative positions.This adds to the versatility of the clamping arrangement. The frictionclamp and the collar clamp are preferably mounted for pivotal movementbetween operative and inoperative positions.

It is also within the scope of the fourth aspect of the invention forthe travelling clamp assembly to be replaced by, for example,tensioners. In a case where there is no travelling clamp assembly, it isthe fixed clamp assembly that includes both a friction clamp and acollar clamp.

According to a fifth aspect of the invention, there is provided apipe-laying vessel including a tower extending upwardly from the vessel,a pipe loading arm for raising a length of pipe from the deck to aposition aligned with the tower; a travelling clamp assembly mounted formovement along the tower, wherein the travelling clamp assembly includesboth a line-up clamp and a pipeline tension bearing clamp.

In use, the travelling clamp assembly can be lowered down the tower tolower the pipeline into the sea.

The pipeline tension bearing clamp is preferably a friction clamp but itmay take other forms such as, for example, a collar clamp. Similarly,the line-up clamp is preferably a friction clamp but it may take otherforms such as, for example, a collar clamp.

The line-up clamp may be mounted on the travelling clamp assembly formovement between an operative and an inoperative position. The movementis preferably pivotal movement.

According to a sixth aspect of the invention, there is provided apipe-laying vessel including a tower extending upwardly from the vessel,a pipe loading arm for raising a length of pipe from the deck to aposition aligned with the tower, and three or more line-up clamps spacedalong the tower for receiving the length of pipe from the loading arm.

By providing three line-up clamps along the tower it is possible toalign especially well a length of pipe that is to be joined to apipeline.

Preferably one of the line-up clamps is mounted on a travelling clampassembly mounted for movement along the tower. In such a case, theposition of one line-up clamp varies but, at the stage when a length ofpipe is being aligned prior to joining to a pipeline, it will be in ahigh position on the tower above the other line-up clamps. The highposition of the clamp helps to line up the top of the length of pipe foran internal line-up clamp that may be lowered into the pipe. Preferablythe line-up clamp is mounted on the travelling clamp assembly in theregion of the top of the assembly and/or above the pipeline tensionbearing clamp.

Preferably the line-up clamp mounted on the travelling clamp assembly isable to support the tension load of the length of pipe when it isclamped to the pipe.

At least one of the line-up clamps, preferably each of the line-upclamps below the line-up clamp mounted on the travelling clamp assembly,includes rollers spaced around the line-up clamp for allowinglongitudinal movement of the length of pipe relative to the clamp whenit is clamped to the pipe.

At least two of the line-up clamps are preferably able to be adjusted ina plane substantially perpendicular to the tower in both orthogonaldirections. That ensures that the desired lateral positioning within thetower of the length of pipe can be achieved.

A vessel of any of the kinds defined above can be designed to operate atconsiderable depths with correspondingly high tension loads. Forexample, the equipment on the vessel is preferably able to handle pipetension loads over 1,000 tonnes and more preferably over 1,500 tonnes,and preferably loads of 2,000 tonnes.

While certain methods of laying a pipeline are already described above,the invention further provides a method of laying a pipeline from apipe-laying vessel, in which the vessel is in any of the forms definedabove for any of the aspects of the invention.

It should be understood that each aspect of the invention may becombined with selected features or all features of one or more of theother aspects of the invention. Indeed in an embodiment of the inventiondescribed below all the aspects of the invention are incorporated in onevessel. Similarly, it should be understood that features described onlyin the context of the vessel may be incorporated in correspondingfeatures of a method of the invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example an embodiment of the invention will now be describedwith reference to the accompanying schematic drawings, of which:

FIG. 1 is a side view from the port side of a stern portion of apipe-laying vessel including a tower;

FIG. 2 is a perspective view, from the port side and from the bow end,of the stern of the vessel shown in FIG. 1;

FIG. 3 is a perspective view, from the starboard side and from the sternend, of the vessel shown in FIG. 1, with a crane and certain other partsof the vessel not shown;

FIG. 4 is a perspective view, from the starboard side and from the bowend, of the vessel, a portion of which is shown in FIG. 1, with a craneand certain other parts of the vessel not shown and with a loading armshown in a fully raised position;

FIG. 5 is a perspective view, from the starboard side and from the bowend, of the loading arm and part of the tower with the loading arm shownin a partly raised position;

FIG. 6 is a perspective view, from the starboard side and from the bowend, of a winch and pulley system for raising and lowering the loadingarm;

FIG. 7 is a perspective view, from the port side and from the bow end,of a travelling block including both a first friction clamp and a firstline-up clamp;

FIG. 8 is a perspective view, from the port side and from the bow end,of a second line-up clamp and mounting system for the clamp;

FIG. 9 is a perspective view to a larger scale than FIG. 8, from thestarboard side and from the bow end, of the second line-up clamp;

FIG. 10 is a perspective view, from the starboard side and from the bowend, of a third line-up clamp;

FIG. 11 is a perspective view, from the starboard side and from thestern end, of a lower clamp assembly mounted on the tower comprising asecond friction clamp and a collar clamp, with the collar clamp shownretracted;

FIG. 12 is a perspective view, from the starboard side and from thestern end, of the lower clamp assembly with the collar clamp in itsoperative position;

FIG. 13 is a side view from the starboard side of the lower clampassembly with both the second friction clamp and the collar clampretracted;

FIG. 14 is a side sectional view from the port side of an assemblystation provided on the vessel;

FIG. 15 is a sectional plan view of the assembly station;

FIG. 16 is a sectional end view from the stern of the assembly station;

FIG. 17 is a sectional perspective view, from the port side and the bowend, of the assembly station;

FIG. 18 is a perspective view, from the starboard side and from thestern end, of the assembly station and a handling system for handlingbulky items;

FIG. 19 is a diagrammatic side view showing a first step in a procedurefor connecting a bulky item to the pipeline;

FIG. 20 is a diagrammatic side view showing a second step in theprocedure;

FIG. 21 is a diagrammatic side view showing a third step in theprocedure;

FIG. 22 is a diagrammatic side view showing a further step in theprocedure; and

FIG. 23 is a diagrammatic side view showing a fifth step in theprocedure.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring first to FIG. 1, a pipe-laying vessel indicated generally bythe reference numeral 1 is a monohull vessel which carries at its sterna pipe-laying tower indicated generally by the reference numeral 2. Alsoshown in FIG. 1, on the starboard side of the vessel, is a craneindicated generally by the reference numeral 3. The vessel is equippedwith a propulsion system which may include a dynamic positioning system.The vessel further includes much other equipment appropriate for theoperation of a pipe-laying vessel, but the description below will belimited mainly to a description of the pipe-laying apparatus and,especially, special features of that apparatus.

Referring also to FIG. 2, the pipe-laying tower 2 is pivotally mountedin a central position, for pivoting movement about a horizontal axisperpendicular to the longitudinal axis of the vessel, on two mountingarms 4P and 4S which project upwardly and rearwardly from the main deckat the stern of the vessel 1. In this illustrated example of theinvention, the tower 2 is able to lay pipe when pivoted at an angle ofabout 45 degrees to the horizontal, as shown in FIG. 2, and when atsteeper angles including the vertical and extending to a position wherethe angle is more than 90 degrees (for example, 96 degrees) so that thetop of the tower projects outwardly beyond the stern of the vessel. Aswill be understood, the tower 2 is used for J-laying of a pipeline.

The vessel 1 is able to lay rigid pipeline that may be supplied to thevessel in straight lengths. Such lengths of pipeline may be weldedtogether in twos, threes or fours on the deck 6 of the vessel to formlonger lengths of pipe which may then be loaded into the tower forjoining to the end of a pipeline being laid. This procedure, which isknown per se is described in more detail in for example U.S. Pat. No.6,524,030, the description of which is incorporated herein by reference.In that particular embodiment two lengths of pipe are welded together toform a double joint string and two double joint strings are subsequentlywelded together to form a quadruple joint string. In the drawings ofthis specification a quadruple joint string is shown but that is ofcourse only one of several possible arrangements.

The tower 2 generally comprises a pair of box girders 5P, 5Sinterconnected by cross girders 7 at intervals, as shown in FIG. 3. Theinclination of the tower is controlled by a pair of beams 8P and 8Swhich are pivotally mounted on the deck 6 and extend upwardly therefrom,passing through the girders 5P and 5S respectively. The girders 5P, 5Sare driven up and down the beams 8P and 8S by rack and pinion drives(not shown) to tilt the tower 2. Although the tower may be intended tooperate only at angles of about 45 degrees or more to the horizontal, itis able to be pivoted down to a substantially horizontal positionthrough the beams 8P and 8S, for example to allow it to be detachedfrom, or attached to, the vessel.

The tower 2 includes a loading arm 9 which is pivotably connected to thegirder 5P of the tower 2 close to the pivotal connection of the girderto the mounting arm 4P. The loading arm can be pivoted, by a rope systemdescribed in more detail below, between a position, shown for example inFIG. 1, in which it lies horizontally over the deck 6 and a position,shown in FIG. 4, in which it lies alongside and on the inside of thegirder 5P. During pipelaying a pipe string 10 is loaded onto the loadingarm while the arm is in its horizontal position. As can be seen in FIG.5, the loading arm is provided with a series of hydraulically operatedclamps 11 along its length. The clamps are mounted so that they can berotated in unison around the loading arm, to enable the pipe string 10to be transferred to clamps mounted for movement along the tower, aswill be described more fully below. At the lower end of the loading arm9, a stopper 12 is provided; the stopper 12 provides an abutment for theend of the pipe string 10 and supports most of the weight of the pipestring 10, so that the clamps 11 do not have to accommodate that load.The clamp at the top of the loading arm 9, referenced 11A in FIG. 5, canbe displaced along the length of the arm so that it engages the topportion of a pipe string 10, for different lengths of string. Theloading arm can accept single, double, triple or quadruple jointstrings.

The loading arm 9 is raised and lowered by a winch and rope system thatwill now be described with reference to FIGS. 5 and 6. The system isactually a dual rope system but, for the purposes of clarity ofillustration, only one rope is shown in the drawings. The systemgenerally comprises a winch 14 having a pair of drums 15A, 15B aroundeach of which a respective rope 16 is wound. Each rope 16 passes fromthe winch 15, which is mounted on a support 17 (also acting as one ofthe cross girders 7) fixed to the tower 2, over pulleys 18 on the girder5P, around a pulley 19 on the loading arm 9, around a pulley 20 on thegirder 5P, around a pulley 21 on the loading arm 9, over a pulley 22 onthe girder 5P, around a pulley 23 on the support 17 and back to afixture on the tower 2 to which the rope is fixed. Along the finalsection of the rope 16, after the pulley 23, an equalizer system 24 isprovided. As already indicated, there are actually two ropes 16 whichpass around their own respective pulleys following the path justdescribed and the equalizer system 24 is connected to both of them. Thepresence of the equalizer system 24 ensures that the tension in eachrope is approximately the same in a manner known per se.

The dual rope system is arranged such that, even if one of the ropes issevered, the other rope is able to continue to raise and lower theloading arm 9, but at a reduced speed of operation.

When the loading arm 9 is in a fully raised position it is held inposition on the girder 5P by one or more clamps (not shown) to provide asecure fastening of the loading arm to the tower 2.

A pipe string 10 raised into position along the tower 2 is received by aline-up clamp system on the tower 2 that will now be described. Thesystem generally comprises a travelling block 30 that includes a firstline-up clamp 33 and a first friction clamp 34 (FIG. 7), a secondline-up clamp 35 (FIGS. 8 and 9) and a third line-up clamp 36 (FIG. 10).The travelling block 30 has laterally extending arms 37 which slidablyengage rails on the girders 5P, 5S to mount the travelling block forsliding movement along the tower. The travelling block 30 is moved alongthe tower by a rope system (not shown) in the tower that is connected toa winch 38 (FIG. 3) on the deck 6 of the vessel in a manner known perse.

While the first line-up clamp 33 is mounted for movement along thetower, the positions along the tower of the second and third line-upclamps are fixed: the second line-up clamp 35 is mounted on the support7 in the regions of the winch 14 (FIG. 3) and the third line up clamp 36is mounted on a support 39 (FIG. 4) lower down the tower.

When a pipe string 10 is received in the tower, the travelling block 30is positioned towards the top of the tower in the position shown in, forexample, FIG. 4. The first line-up clamp 33 is pivotally mounted on thetravelling block pivotable about a horizontal axis through 90 degreesbetween a retracted position shown in FIG. 7 in which the clamp isdisposed in a vertical plane and an operative position in which theclamp is disposed in a horizontal plane. As shown in FIG. 7, the clampitself also has an open position, shown in FIG. 7, in which a jaw 40 ofthe clamp is retracted to an open position, and a closed position inwhich the jaw 40 is pivoted through an angle of about 90 degrees toclose the central opening through the clamp.

The first friction clamp 34, which may be of a design known per se ismounted on the travelling block 30 below the line-up clamp 33 and ismovable between an open position, shown in FIG. 7, and a closed positionin which pads in the clamp are pressed inwardly to grip a pipe stringheld in the clamp. The first friction clamp 34 is pivotally mounted atpivots 41 on the travelling block and is retractable from an operativeposition shown in FIG. 7 to a retracted position by operation of ahydraulic cylinder 42 which causes the support on which the firstfriction clamp 34 is pivotally mounted to pivot rearwardly therebymoving the first friction clamp 34 rearwardly. The first friction clamp34 is also provided internally with a slewing mechanism of a kind knownper se that enables a pipe string held by it to be rotated about itslongitudinal axis by ±190 degrees. The slewing mechanism is not shown inthe drawings.

The second line-up clamp 35, shown in FIGS. 8 and 9, is carried on anextensible arm 43 that is pivotally mounted at pivot 44 for pivotingmovement about an axis parallel to the longitudinal axis of the tower 2.Pivoting movement of the clamp 35 is controlled by a hydraulic cylinder47. Like the line-up clamp 33, the clamp 35 has a pivotally mounted jaw45 movable between a closed position, shown in FIGS. 8 and 9, and anopen position by a hydraulic cylinder 46. The clamp 35 differs from theclamp 33 in that the clamp 35 allows longitudinal movement of the pipestring 10 through the clamp and for this purpose a plurality ofangularly spaced rollers 48 are provided which are resiliently pressedagainst the pipe string 10 but are able to allow the passage of the pipestring 10, including for example the welded joints of a stringcomprising four pipe lengths (a quadruple string). The rollers 48 arealso hydraulically retractable away from the pipe-laying path.

The third line-up clamp 36 shown in FIG. 10 is of similar constructionto the second line-up clamp 35 and operates in a similar way. The clamp36 is carried on an extensible arm 50 that is pivotally mounted on thetower in a similar manner to the second line-up clamp 35. The clamp 36allows longitudinal movement of the pipe string 10 through the clamp andfor this purpose a plurality of angularly spaced rollers 51 are providedwhich are resiliently pressed against the pipe string 10 but are able toallow the passage of the pipe string, including for example weldedjoints. The rollers 51 are also retractable away from the pipe-layingpath.

In addition to the clamp system described above, the tower 2 is alsoprovided, at its lower end, with a lower clamp assembly 60 which is ableto engage the upper end of the pipeline to which the pipe string 10 isto be joined. The lower clamp assembly 60 is mounted on a framework 61at the bottom of the tower below the axis of pivoting of the tower 2 onthe mounting arms 4P and 4S as shown for example in FIGS. 1, 3 and 11.The lower clamp assembly includes both a collar clamp 62 and a secondfriction clamp 63 and each of the clamps is independently retractableout of the pipelaying path. Each of these clamps is commonly referred toas a “fixed clamp” or a “hang-off clamp”.

The collar clamp 62 is supported on two rear legs 64 and two front legs65. The rear legs 64 are pivotally mounted on a horizontal beam 61A ofthe framework 61 and the front legs 65 are detachably secured to theframework 61. Thus, with the front legs 65 detached, the collar clampcan be retracted to the position shown in FIGS. 11 and 13. A pair ofhydraulic cylinders 67 are provided to effect the pivoting movement ofthe legs. The collar clamp 62 may itself be of a design known per se andmay be provided with a hydraulically actuated jaw 66 shown in an openposition in FIGS. 11 to 13.

The second friction clamp 63 comprises both a table portion 68 and clampportion 69 upstanding therefrom. Both the table portion 68 and the clampportion 69 are split into two halves 68P, 68S and 69P, 69S, with theplane of the split being a vertical plane containing the longitudinalaxis of the vessel. The ability of the clamp 63 to split enables thepipeline to pass freely through the second friction clamp even if it isnot retracted and even allows a bulky item to pass through the secondfriction clamp. The clamp portion 69 of the second friction clamp 63 maybe of a design known per se and may for example include friction padspressed by hydraulic pressure against the pipeline.

The table portion 68 of the second friction clamp 63 is pivotallyconnected at its rear end to the horizontal beam 61A and is supportedvertically by that beam and another beam 61B on the opposite side of thepipelaying path. A pair of hydraulic cylinders 70 are provided forpivoting the table portion 68, and therefore the entire second frictionclamp 63, between its operative position shown in FIGS. 11 and 12 and aretracted position shown in FIG. 13. In normal operation, the secondfriction clamp 63 is in its operative position and the collar clamp 62is retracted, as shown in FIG. 11.

A safety clamp (not shown) may be provided within the table portion 68.Such a clamp can be used to grip the pipeline in the event of anyslippage through the lower clamp assembly being detected. Thus thesafety clamp can provide a contingency system.

The vessel is provided with a welding assembly station 100 at whichpipeline assembly operations are carried out. The assembly station 100generally comprises a cabin 101 which is slidably mounted by wheels 102on rails 103 provided on the deck 6 as shown for example in FIGS. 14 and16. The cabin 101 can thereby be driven between an operative positionshown in FIG. 14 in which the pipeline path passes through the cabin anda retracted position, shown for example in FIG. 2, in which the cabin101 is clear of the pipeline path. As shown in the drawings the cabin issupported on a frame 104 which serves to raise the floor of the cabin toa level such that the height within the cabin for the joint (fieldjoint) of the bottom of a pipe string to the top of a pipeline is at thelevel of the axis of pivoting of the tower 2 on the mounting arms 4P and4S. By arranging for that axis of pivoting to pass through the locationof the joint of the new pipe string with the existing pipeline, itbecomes much easier to operate across a range of inclinations of thetower 2. For example, whilst FIG. 14 shows a pipe string 10 being joinedto the top of a suspended pipeline P with the tower 2 vertical, FIG. 17shows the junction being made with the tower 2 at an angle of 45 degreesto the horizontal and it can be seen that the location of the joint isstill within the cabin.

As shown in FIGS. 14 to 17, the aft end wall of the cabin and portionsof the roof and floor of the cabin are cut away to form a central endslot 105 in which the pipeline can be accommodated as the cabin is movedfrom its retracted position to its operative position. Mounted insidethe roof of the cabin is a carousel 106 carrying equipment for use inwelding the pipe string 10 to the pipeline P. The carousel may be ahorseshoe shape to facilitate its introduction around the pipelayingpath and may be as described in U.S. Pat. No. 6,313,426, the descriptionof which is incorporated herein by reference. An overhead crane 108,diagrammatically shown in FIGS. 15 and 16 is provided in the top of thecabin 101 and runs on rails 107 extending along the length of the cabin.Other rails, not shown, may also be provided on the floor of the cabin,extending from the aft end of the cabin, which may be openable, to eachside of the location of the field joint.

The portion of the end slot 105 in the aft end wall of the cabin and theportions of the slot 105 in the roof and floor of the cabin haveassociated closure members (not shown) to enable the slot 105 to beclosed over after movement of the assembly station between its retractedand operative positions. Thus the cabin is weatherproof except when itis being used.

The vessel is also equipped with a handling system for enabling bulkyitems to be included along the pipeline when required. Bulky items suchas valve blocks and PLEMs are sometimes required to be laid and it isdesirable that their laying should not unduly disrupt or delay thepipelaying. The handling system for handling bulky items generallycomprises an extension frame 110 and a tilting frame 111, shown, forexample, in FIG. 18. The extension frame 110 includes a pair of legs 112carrying wheels at their bottom ends that engage the rails 103 on whichthe cabin 101 is also mounted. The extension frame also includes a pairof longitudinal beams 113 that extend horizontally towards the stern ofthe vessel, being supported at their bow ends by the legs 112 and attheir stern ends carry wheels which engage rails 114 on the roof of thecabin 101. In that way the extension frame 110 is able to move along thedeck 6 of the vessel and can move in unison with the cabin 101, but alsoindependently of the cabin 101.

The tilting frame 111 is mounted over the extension frame 110 and ispivotally connected at its stern end to the extension frame at locationsindicated by reference numeral 116. In a preferred arrangement thelocations 116 can be adjusted to facilitate operations at differenttower angles and/or for different shapes of bulky item. One or morehydraulic cylinders (not shown) are provided for tilting the tiltingframe 111 relative to the extension frame. As can be seen in FIG. 18,the stern end of the tilting frame incorporates a slot 117 whichoverlies an open ended part of the extension frame 110. In FIG. 18, theextension frame 110 and tilting frame 111 are shown in their mostforward (advanced towards the pie-laying path) positions relative to thecabin 101, but during normal operation when no bulky item is being addedto the pipeline the extension frame 110 and tilting frame 111 are in theposition shown in, for example, FIG. 14. The slot 117 in the tiltingframe 111 and the open end of the extension frame 110 allow free passageof the pipe string 10, even when the tower is tilted to an angle of 45degrees.

The sequence of operations involved in a cycle of adding one pipe string10 to the pipeline P will now be described, taking first the case of anordinary cycle of operations. Thereafter special cycles of operationswill be described, including one in which a bulky item is added to thepipeline.

An ordinary cycle of operation will be described starting from theposition where the loading arm 9 has just been raised into the tower 2and locked thereto and the pipeline P has just been lowered with a newpipe string added thereto and the travelling block 30 is at the bottomof the tower 2 above the lower clamp assembly 60. The second frictionclamp 63 is closed holding the tension in the pipeline P and the collarclamp 62 is in its retracted position. At this stage the cabin 101 hasbeen moved forwards away from the tower.

The travelling block 30 is first raised to the top of the tower 2 andthe clamps 11 and the loading arm 9 are rotated to load a new pipestring 10 into the pipelaying path (firing line) in the tower 2. At thetop of the tower 2, is an internal line-up system 75, including a winchand an internal line-up clamp which is lowered inside the new pipestring in a manner known per se.

The line-up clamps 35 and 36 are in a retracted position away from thefiring line as the travelling block is moved up the tower. As soon asthe travelling block passes each of the clamps 35, 36, they are deployedinto their operative positions. When the clamps 11 on the loading arm 9are moved round as the loading arm 9 is rotated, the new pipe string isintroduced into the clamps 33, 35 and 36 which are then closed aroundthe new pipe string 10 the clamp 33 also being in its operativepositions by this time. The line-up clamp 33 bears the weight of the newpipe string 10. The clamps 11 are then opened, releasing the new pipestring from the loading arm 9.

The cabin 101 is then moved rearwardly towards the tower and the top ofthe pipeline P and the bottom of the new pipe string 10 received withinthe cabin 101, entering through the slot 105 in the cabin, the slotbeing closed after the cabin 101 has reached its final position. Theprecise position of the new pipe string 10 can then be adjusted to moveit into exactly the position required for welding to the pipeline P. Thelongitudinal position of the pipe string 10 along the tower 2 can beadjusted by movement of the travelling block and the lateral position ofthe pipes relative to the firing line can be adjusted in both orthogonaldirections by the line-up clamps, and the internal line-up clamp. Itwill be understood that the line-up clamps 35, 36 allow freelongitudinal movement of the pipe string 10 through them but are able toapply appropriate transverse loads to deform the pipe string 10 ifnecessary and line it up.

Once the pipe string 10 is correctly positioned it is welded to thepipeline P and NDT and coating steps also carried out on the joint. Theinternal line-up clamp is then raised, the line-up clamp 33 opened andthen retracted and the first friction clamp 34 on the travelling block30 closed to grip the top of the pipe string 10.

The pipeline path (firing line) passes through the axis of pivoting, ofthe tower 2 on the mounting arms 4P, 4S, and the position of the fieldjoint between the new pipe string 10 and the pipeline P is on that axis.Thus the position of the field joint does not change as the tower ispivoted.

When the first friction clamp 34 is fully engaged with the top of thepipe string 10, the second friction clamp 63 is opened so the pipelineis then suspended from the first friction clamp 34. The travelling block30 is then lowered and at the same time the cabin 101 is moved forwardsaway from the tower so that the travelling block 30 is able to belowered by the main winch 38 to the level of the field joint. As thetravelling block 30 is lowered each of the line up clamps 35, 36 areretracted to allow passage of the travelling block 30. Once thetravelling block has reached a position in which the top of the pipestring 10 is at the location of the top of the pipeline P at thebeginning of the cycle, the travelling block 30 stops and the secondfriction clamp 63 is closed on the pipeline P and takes the tension loadof the pipeline. When the second friction clamp 63 is fully engaged withthe pipeline P, the first friction clamp 34 is opened releasing the loadfrom the travelling block.

After the clamps 11 on the loading arm 9 are opened, releasing the pipestring 10 from the loading arm 9, the fastening of the loading arm tothe girder 5P is released and the winch 14 operated to lower the loadingarm down to its horizontal position on the deck 6. A new pipe string 10is then loaded onto the loading arm, being held by the clamps 11 whichhave been rotated back to their position at the beginning of the cycleand the loading arm then raised by the winch 14 back to a positionadjacent the girder 5P and fastened thereto. The lowering and raising ofthe loading arm 9 takes place in parallel with the welding of the pipestring to the pipeline and the lowering of the pipeline so that the newpipe string is ready to be rotated into position along the firing lineonce the travelling block 30 has been lowered.

In the sequence of operations described above, the second friction clamp63 is used and not the collar clamp 62. In an alternative procedure, thesecond friction clamp 63 is retracted and the collar clamp 62 is pivotedinto its operational position. The collar clamp 62 is then opened andclosed at the same times as the second friction clamp 63 in the sequenceof operations described above. As will be understood, the collar clamp62 holds the pipeline P by engaging the underside of a collar formed onthe pipeline P and, when it is used, each new pipe string 10 is providedwith a collar at the appropriate position for engagement by the collarclamp 62.

A vessel of the kind described above can be designed to accommodate veryhigh pipeline tension loads, enabling it to operate in very deep water.For example, the vessel may be able to lay pipeline while accommodatinga tension load in the tower of over 1,000 tonnes. The second frictionclamp 63 and/or the collar clamp 62 may be able to accommodate a tensionload of over 1,500 tonnes (in a preferred embodiment 2,000 tonnes) andthe travelling block and the first friction clamp 34 may be able toaccommodate a tension load of over 1,000 tonnes (in a preferredembodiment 1,500 tonnes) during operation.

If the vessel is to operate with the tower 2 at an angle of more than 90degrees to the horizontal, then it may be necessary to provide anadditional facility to control movement of the loading arm 9 beyond a 90degree angle. Such a facility may for example be provided by anadditional winch which would only be required to handle the low loadsneeded to control movement of the loading arm 9 in the region of avertical position.

As is already known for pipe-laying vessels, the vessel is also equippedwith appropriate equipment to enable the pipeline P to be abandoned fromand recovered to the vessel.

If a bulky item is to be added to the pipeline P, the loading arm 9 isnot used to load the bulky item but, instead the handling systemincluding the extension frame 110 and the tilting frame 111 are used ina manner that will now be described with reference to FIGS. 19 to 23.

With the cabin 101 in its forward position clear of the tower 2 and withthe extension frame 110 in its forward position as shown in FIG. 19, abulky item assembly 120, comprising in this example a quadruple pipestring 121 incorporating a bulky item 122 shown as a box shape is loadedby a crane, for example the crane 3, onto the extension frame 110leaving the parts in the positions shown in FIG. 20.

The extension frame 110 (and therefore also the tilting frame 111 andthe bulky item assembly 120) is then moved aft to the position shown inFIG. 21, with the wheels on the legs 112 travelling along the rails 103and the wheels on the stern ends of the beams 113 of the extension frame110 travelling along the rails 114 on the roof of the cabin 101. Thatbrings the parts into the positions shown in FIG. 21. The extensionframe 110 is then fastened to the cabin 101.

The cabin is then moved aft to the position shown in FIG. 22 taking theextension frame 110, the tilting frame 111 and the bulky item assembly120 with it. Also, the bulky item assembly 120 is skidded along theextension frame 110 to lie over the tilting frame 111 and is secured toit. As can be seen from FIG. 22 the cabin 101 is moved far enough aftthat the pivot locations 116 are close to the pipelaying path in thetower 2.

The tilting frame 111 is then tilted about the extension frame 110 atthe pivot locations 116 using the hydraulic cylinders until the tiltingframe 111 extends parallel to the tower 2. In this position the pipestring 121 is disposed along the normal pipestring path within the tower2 and the pipe string 121 can be engaged by the line-up clamps 33, 35and 36, in substantially the same manner as the pipe string 10. Also thebulky item 122 may engage the rails on the girders 5P, 5S of the tower2.

Once the pipe string 121 is securely held in the tower 2, the fasteningto the tilting frame 111 is released and the tilting frame returned toits horizontal position on the extension frame 110. The cabin 101 canthen be moved further aft and, if necessary, the bulky item lowered ashort distance down the tower to bring the field joint between the bulkyitem assembly 120 and the pipeline to the usual location in the cabin101. Once the various operations associated with making the field jointare complete, the cabin can be retracted towards the bow of the vessel,the pipe string 121 can be engaged by the first friction clamp 34 on thetravelling block 30, the line-up clamps 33, 35 and 36 can be opened andretracted, the second friction clamp 63 can be released and split intoits two halves and the pipeline with the bulky item assembly 120 nowdefining its upper end lowered by the travelling block 30 down to theusual lower position of the travelling block 30 at the level where thefield joint is made. The second friction clamp 63 can then be returnedto its normal unsplit position and operated to grip the upper end of thepipe string 121 with the bulky item 122 now deployed in the water clearof the clamp 63.

The description above of the operation of the bulky item handling systemhas been provided for the case where the tower 2 is inclined at an angleof about 45 degrees to the horizontal but it will be understood that theoperation can be carried out in the same way for steeper angles of thetower 2, including the case where the tower is vertical.

In the procedure described immediately above, the bulky item assembly120 is first placed towards the bow end of the extension frame 110 andis then skidded onto the tilting frame 111. An alternative possibilityis to place the assembly 120 in its final position on the tilting frame111 at the outset.

Whilst one particular form of vessel has been described above withreference to the drawings, it will be understood that many modificationsmay be made to that embodiment and some of these modifications arementioned below.

In the described embodiment a travelling block 30 is provided forlowering the pipeline P into the water, but it should be understood thatother systems, for example, using tensioners, may also be employed.Also, in the described embodiment the loading arm 9 is pivotally mountedin the region of the bottom of the tower but an alternative arrangementwould be for it to be in the form of a strongback and for the end of theloading arm nearer to the tower when the arm is horizontal to be raisedup the tower. Furthermore, in the described embodiment, the tower isable to operate at a range of angles but it is also possible for thetower to be fixed, for example, in a vertical position. In FIGS. 3 and4, the tower 2 is shown with a stinger 80 at its bottom, this being anoption (usually preferred). The construction of the stinger 80 may be ofa kind known per se.

Where in the foregoing description, integers or elements are mentionedwhich have known, obvious or foreseeable equivalents, then, suchequivalents are herein incorporated as if individually set forth.Reference should be made to the claims for determining the true scope ofthe present invention, which should be construed so as to encompass anysuch equivalents. It will also be appreciated by the reader thatintegers or features of the invention that are described as preferable,advantageous, convenient or the like are optional and do not limit thescope of the independent claims.

1-33. (canceled)
 34. A pipe-laying vessel including a deck and a towerextending upwardly from the vessel, a pipe loading arm for raising alength of pipe from the deck to a position aligned with the tower, asupport assembly for supporting a bulky item to be accommodated alongthe pipeline, the support assembly including a tiltable frame pivotablebetween a first load receiving position and a second tilted position inwhich the frame extends approximately parallel to the tower, and drive amechanism for pivoting the tiltable frame between its first and secondpositions and wherein the vessel further includes a welding assemblystation cabin in a region of a lower portion of the tower, the supportassembly being disposed on top of the cabin.
 35. A vessel according toclaim 34, in which the tiltable frame extends approximately horizontallywhen it is in the first load receiving position.
 36. A vessel accordingto claim 34, in which the support assembly is a movable support on whichthe tiltable frame is pivotally mounted.
 37. A vessel according to claim36, in which the movable support assembly is movable towards and awayfrom the tower.
 38. A vessel according to claim 34, in which at leastpart of the support assembly rests on and is supported by the cabin. 39.A vessel according to claim 34, in which the cabin is movable towardsand away from the tower.
 40. A vessel according to claim 39, in whichthe support assembly is movable in unison with the cabin.
 41. A vesselaccording to claim 39, in which the support assembly is movableindependently of the cabin.
 42. A vessel according to claim 39, in whichthe cabin and the support assembly are mounted at least partly on acommon set of rails.
 43. A vessel according to claim 39, in which thesupport assembly is mounted at least partly on a set of rails on theroof of the cabin.
 44. A vessel according to claim 34, in which thesupport assembly comprises a first frame, the tiltable frame defining asecond frame pivotally mounted on the first frame and pivotable betweenthe first load receiving position and the second tilted position.
 45. Avessel according to claim 34, in which the tower is pivotally mountedand is able to operate over a range of angles to the horizontal.
 46. Avessel according to claim 34, in which a travelling clamp assembly ismounted for movement along the tower and a fixed clamp assembly islocated in the region of a lower portion of the tower.
 47. A vesselaccording to claim 46, in which the travelling clamp assembly and/or thefixed clamp assembly includes both a friction clamp and a collar clamp,each of the friction clamp and the collar clamp being movable between anoperative position, in which it is adjustable between a clamping and areleased position, and an inoperative position.
 48. A vessel accordingto claim 46, in which the travelling clamp assembly includes both aline-up clamp and a pipeline tension bearing clamp.
 49. A vesselaccording to claim 34, including three or more line-up clamps spacedalong the tower for receiving the length of pipe from the loading arm.50. A vessel according to claim 34, in which a rope and winch system isprovided for moving the pipe loading arm to raise the length of pipe,the rope and winch system including two ropes operating in parallelwhereby movement of the loading arm can still be effected by the systemin the event that one of the two ropes is inoperative.
 51. A vesselaccording to claim 34, in which the axis of pivoting of the tiltableframe is in the region of the tower.
 52. A pipe-laying vessel includinga deck and a tower extending upwardly from the vessel, a pipe loadingarm for raising a length of pipe from the deck to a position alignedwith the tower, a welding assembly station cabin in a region of a lowerportion of the tower, a movable support assembly for supporting a bulkyitem to be accommodated along the pipeline, the movable support assemblybeing disposed on top of the cabin.
 53. A pipe-laying vessel including adeck and a tower extending upwardly from the vessel, a pipe loading armfor raising a length of pipe from the deck to a position aligned withthe tower, wherein a rope and winch system is provided for moving thepipe loading arm to raise the length of pipe, the rope and winch systemincluding two ropes operating in parallel whereby movement of theloading arm can still be effected by the system in the event that one ofthe two ropes is inoperative.
 54. A vessel according to claim 53, inwhich the pipe loading arm is pivotally mounted about a pivot in aregion of the bottom of the tower for allowing the loading arm to pivotbetween a lower, substantially horizontal position and a raised positionin which the loading arm is substantially parallel to the longitudinalaxis of the tower.
 55. A vessel according to claim 53, in which the tworopes follow adjacent paths.
 56. A vessel according to any of claim 53,in which the two ropes are wound on a same winch drum or adjacent winchdrums driven by a same drive.
 57. A method of laying a pipeline from apipe-laying vessel, the method comprising the following steps: providinga vessel with an upwardly extending tower, raising a length of pipe on apipe loading arm from a deck of the vessel to a position aligned withthe tower, welding the length of pipe to an end of a pipeline suspendedfrom the vessel, lowering the pipeline with the length of pipe weldedthereto, resting a bulky item to be added to the pipeline on a tiltableframe, tilting the frame to a position extending approximately parallelto the tower, transferring support for the bulky item from the tiltableframe to one or more devices in the tower, and connecting the bulky itemto the pipeline suspended from the vessel, and wherein the welding ofthe length of pipe to the end of the pipeline is carried out in awelding assembly station cabin disposed in a region of a lower portionof the tower.
 58. A method according to claim 57, in which the tiltableframe extends approximately horizontally when it receives the bulkyitem.
 59. A method according to claim 57, in which the tiltable frame istilted from a position overlying a top of the cabin to the positionextending approximately parallel to the tower.
 60. A method according toclaim 57, further including the step of moving the cabin towards thetower.
 61. A method according to claim 59, in which the cabin and thetiltable frame are moved towards the tower in unison.
 62. A methodaccording to claim 57, in which the bulky item is rested on a movablesupport assembly that includes the tiltable frame, the support assemblyis moved towards the tower and the cabin in a first step and is moved inunison with the cabin towards the tower in a subsequent step.
 63. Amethod of laying a pipeline from a pipe-laying vessel, the methodcomprising the following steps: providing a vessel with an upwardlyextending tower, raising a length of pipe on a pipe loading arm from adeck of the vessel to a position aligned with the tower, welding thelength of pipe to an end of a pipeline suspended from the vessel, thewelding being carried out in a welding assembly station cabin disposedin a region of a lower portion of the tower, lowering the pipeline withthe length of pipe welded thereto, resting a bulky item to be added tothe pipeline on a movable support assembly disposed on top of the cabin,moving the movable support assembly towards the tower, transferring thebulky item from the movable support assembly into the tower, andconnecting the bulky item to the pipeline suspended from the vessel. 64.A method of laying a pipeline from a pipe-laying vessel, in which thevessel is as claimed in claim 34.